1. Is that a good time to dive Cozumel?
2. what are the best reef/dive sites not to be missed in Cozumel?
3. whats the best dive OP in Cozumel for the above.
To answer your questions:
1. There are no BAD times to dive Cozumel. Early March will have some of the coolest water temps there, but that means 78-80 degrees F, hardly what I'd call chilly. Maybe a 3 mil wetsuit.
2. Based on your criteria, pretty much all of the sites in the Marine Park can fit your desires. I'd avoid the Northern Reefs, Barracuda and San Juan - the current can be ripping up there, and those reefs are mostly enjoyed by divers looking for the underwater equivalent of Mr. Toad's Wild Ride. And Devil's Throat and Punta Sur are probably not really your cup of tea. But Columbia Shallows, Columbia Deep, all of Palacar - Gardens, Bricks, Caves and Horseshoe, Delilah, Tormentos, Cedral, and others all fit your bill. Just let the DM pick your spots.
3. There is no "Best Dive Op" in Cozumel - it all depends on what your experience level is and what your personal desires are - Big Boat vs. fast six-pack? Big tanks (steel 120s) and long, long bottom times vs. AL 80s and 45-50 minute dives? Long SI on the beach or at a beach club vs, 45 minute SI on the boat? Low cost operator vs. high-end valet service? DMs that babysit you vs. DMs that give you a lot of freedom to do your own thing? There are a ton of very good Dive Ops on Cozumel, and a lot of cheerleaders for each. Aldora, Blue XTSea, Tres Pelicanos, Liquid Blue, Dive with Martin, and at least a dozen others that have been mentioned on other posts. Do your own research..
One last thing. You mentioned staying close to the dive sites. Nearly all the dive sites are at the south end of the island, and pretty much all the Resorts down there are All Inclusives. If you've never been to Cozumel before, be aware that one of Cozumel's big draws (in addition to great diving) for many devoted Cozaholics is the wealth of great restaurant options, and nearly all of those are in town. I, and many others, refuse to do AI on Cozumel simply because there are too many good restaurants that we'd miss. But even staying in town or near town, it's not a terribly long boat ride to the dive sites - 20-35 minutes at most, and staying in town means easy walking access to a lot of very good dining options. Consider that in deciding where to stay.
Pretty much any site can give you what you are looking for. I find Columbia and Palancar the best for a variety of life, but each site has something special. Last March there were spotted eagle rays everywhere. 
